Added Marker to JSON layout.

[seb-droig]

I've been scouring the Internet for clues on how to do it with Logback, but ultimately decided to fix this at the source.

I've tried solving my problem by implementing a class that inherits from ch.qos.logback.contrib.json.classic.JsonLayout, but I lost control on where to put the Marker field since it internally uses a LinkedHashMap so it wound up at the end, which looks a bit weird. My implementation worked, but I figured others might benefit from this functionality, so that's why I created this PR. I've tried to put the Marker field where I thought it made sense (after the log level), but you can obviously change this at your discretion.

Hopefully, you'll find it worthwhile to create a new release with this.

Thanks for maintaining Logback, it's usually my go-to logging implementation.